For removing a nitrogen oxide in a combustion exhaust gas of an internal combustion engine, such as a marine diesel engine, an ammonia selective catalytic reduction method is mainly used. The ammonia selective catalytic reduction method is such a method that a denitration catalyst containing vanadium and titania as a major component is used as a catalyst, and ammonia is used as a reducing agent.
However, in an internal combustion engine, such as a marine diesel engine, a C heavy oil or the like is combusted in the internal combustion engine, as different from an automobile diesel engine, and since a C heavy oil or the like contains a sulfur component, a sulfur oxide (SOx) is generated along with a nitrogen oxide (NOx) in the combustion exhaust gas. In the case where the combustion exhaust gas of this type is denitrated by an ammonia selective catalytic reduction method, a sulfur oxide and ammonia are reacted with each other in the combustion exhaust gas to form ammonium sulfate ((NH4)2SO4). Furthermore, in an internal combustion engine, such as a marine diesel engine, the temperature of the exhaust gas after passing through a supercharger becomes as low as approximately 250° C., and thus there may be a problem that ammonium sulfate formed through reaction of the sulfur oxide in the exhaust gas and ammonia as a reducing agent is deposited on the exhaust path, which causes clogging of the heat exchanger.
On the other hand, as a reductive removing method using a reducing agent other than ammonia, for example, PTL 1 shown below describes a method of using a catalyst containing a metal supported on zeolite and an alcohol as a reducing agent.
PTL 2 shown below describes that a denitration catalyst is disposed in an exhaust gas processing flow path that is branched into two paths, and while one of the exhaust gas processing flow path is closed to terminate the supply of the exhaust gas, whereas the exhaust gas processing is continued in the other of the exhaust gas processing path, the denitration catalyst layer of the exhaust gas processing path where the supply of the exhaust gas is terminated is heat-treated on the spot at a temperature of from 350 to 800° C., thereby recovering the deteriorated denitration capability thereof.